When stepping on the brakes at any pressure other than the most gentle, there is a loud "clunk" from the rear of the car, and a similar clunk when the pedal is released. With help, I was able to identify that the caliper assembly is moving and clunking against something, but I don't know what.

Suspension mods are Pedders Street Xa package and differential bushings with Pfadt trailing arms. I have been running fine on the setup for several weeks and then this noise started. Thanks for the help in advance.

Sorry, I'm sure my terminology is inaccurate. I think it could be described as the part that includes the pads/calipers and what they are mounted to. everything seems to be bolted in correctly, and everything was fine for several weeks after the install. I grant I could be mistaken, and if so, what else could cause that type of clunk?